Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970's there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.
Group IBIIIA VIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn(1-x) Gax (SySe(1-y))k, where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. Absorbers containing Group IIIA element Al and/or Group VIA element Te also showed promise. Therefore, in summary, compounds containing: i) Cu from Group IB, ii) at least one of In, Ga, and Al from Group IIIA, and iii) at least one of S, Se, and Te from Group VIA, are of great interest for solar cell applications.
The structure of a conventional Group IBIIIA VIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te)2 thin film solar cell is shown in FIG. 1. The device 10 is fabricated on a substrate 11, such as a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. The absorber film 12, which comprises a material in the family of Cu(In,Ga,Al)(S,Se,Te)2, is grown over a conductive layer 13 or contact layer, which is previously deposited on the substrate 11 and which acts as the electrical contact to the device. The substrate 11 and the conductive layer 13 form a base 13A on which the absorber film 12 is formed. Various conductive layers comprising Mo, Ta, W, Ti, and stainless steel etc. have been used in the solar cell structure of FIG. 1. If the substrate itself is a properly selected conductive material, it is possible not to use the conductive layer 13, since the substrate 11 may then be used as the ohmic contact to the device. After the absorber film 12 is grown, a transparent layer 14 such as a CdS, ZnO or CdS/ZnO etc. stack is formed on the absorber film. Radiation 15 enters the device through the transparent layer 14. Metallic grids (not shown) may also be deposited over the transparent layer 14 to reduce the effective series resistance of the device. The preferred electrical type of the absorber film 12 is p-type, and the preferred electrical type of the transparent layer 14 is n-type. However, an n-type absorber and a p-type window layer can also be utilized. The preferred device structure of FIG. 1 is called a “substrate-type” structure. A “superstrate-type” structure can also be constructed by depositing a transparent conductive layer on a transparent superstrate such as glass or transparent polymeric foil, and then depositing the Cu(In,Ga,Al)(S,Se,Te)2 absorber film, and finally forming an ohmic contact to the device by a conductive layer. In this superstrate structure light enters the device from the transparent superstrate side. A variety of materials, deposited by a variety of methods, can be used to provide the various layers of the device shown in FIG. 1.
In a thin film solar cell employing a Group IBIIIA VIA compound absorber, the cell efficiency is a strong function of the molar ratio of IB/IIIA. If there are more than one Group IIIA materials in the composition, the relative amounts or molar ratios of these IIIA elements also affect the properties. For a Cu(In,Ga)(S,Se)2 absorber layer, for example, the efficiency of the device is a function of the molar ratio of Cu/(In+Ga). Furthermore, some of the important parameters of the cell, such as its open circuit voltage, short circuit current and fill factor vary with the molar ratio of the IIIA elements, i.e. the Ga/(Ga+In) molar ratio. In general, for good device performance Cu/(In+Ga) molar ratio is kept at around or below 1.0. As the Ga/(Ga+In) molar ratio increases, on the other hand, the optical bandgap of the absorber layer increases and therefore the open circuit voltage of the solar cell increases while the short circuit current typically may decrease. For techniques (such as evaporation) that yield relatively uniform Ga distribution through the thickness of the CIGS absorber layer, the optimum Ga/(Ga+In) molar ratio has been found in the 0.2-0.3 range. It is important for a thin film deposition process to have the capability of controlling both the molar ratio of IB/IIIA, and the molar ratios of the Group IIIA components in the composition. It should be noted that although the chemical formula is often written as Cu(In,Ga)(S,Se)2, a more accurate formula for the compound is Cu(In,Ga)(S,Se)k, where k is typically close to 2 but may not be exactly 2. For simplicity we will continue to use the value of k as 2. It should be further noted that the notation “Cu(X,Y)” in the chemical formula means all chemical compositions of X and Y from (X=0% and Y=100%) to (X=100% and Y=0%). For example, Cu(In,Ga) means all compositions from CuIn to CuGa. Similarly, Cu(In,Ga)(S,Se)2 means the whole family of compounds with Ga/(Ga+In) molar ratio varying from 0 to 1, and Se/(Se+S) molar ratio varying from 0 to 1.
If there is more than one Group VIA material or element in the compound, the electronic and optical properties of the Group IBIIIA VIA compound are also a function of the relative amounts of the Group VIA elements. For Cu(In,Ga)(S,Se)2, for example, compound properties, such as resistivity, optical bandgap, minority carrier lifetime, mobility etc., depend on the Se/(S+Se) ratio as well as the previously mentioned Cu/(In+Ga) and Ga/(Ga+In) molar ratios. Consequently, solar-to-electricity conversion efficiency of a CIGS(S)-based solar cell depends on the distribution profiles of Cu, In, Ga, Se and S through the thickness of the CIGS(S) absorber. For example, the data in FIG. 2 schematically shows a typical distribution profile for the Ga/(Ga+In) molar ratio for a Cu(In,Ga)Se2 absorber layer formed by a two-stage process involving selenization of a metallic precursor comprising Cu, In and Ga. Examples of such two-stage processes may be found in various publications. For example, U.S. Pat. No. 6,048,442 discloses a method comprising sputter-depositing a stacked precursor film containing a Cu—Ga alloy layer and an In layer to form a Cu—Ga/In stack on a metallic back electrode layer in the first stage of the process, and then reacting this precursor stack film with one of Se and S to form the absorber layer during the second stage of the process. U.S. Pat. No. 6,092,669 describes the sputtering-based equipment for producing such absorber layers.
As stated above, FIG. 2 shows a typical Ga/(Ga+In) molar ratio data taken from a CIGS layer grown by a two-stage process. As can be seen from this figure one problem faced with the selenization type or two-stage processes is the difficulty to distribute Ga through the thickness of the CIGS absorber layer formed after the reaction of the metallic precursor film with Se. The vertical “y” axis in FIG. 2 shows the Ga/(Ga+In) molar ratio in arbitrary units (A.U.). Along the horizontal “x” axis, “0” refers to the surface of the CIGS absorber layer and “t” refers to the location of the “CIGS film/contact layer” interface. In other words the thickness of the CIGS layer of FIG. 2 is about “t” micrometers. It should be noted that the same phenomenon has been observed for films comprising Al also. For example, when two stage techniques were used to grow Cu(In,Al)Se2 (CIAS) films, Al distribution was found to be very similar to the Ga distribution depicted in FIG. 2. Therefore, although CIGS is used as an example to describe certain preferred embodiments, other embodiments may include CIAS layers or CIGAS layers comprising both Ga and Al. Further, the films may comprise Group IB materials other than Cu (such as Ag) and Group VIA materials other than Se (such as S).
As can be seen from FIG. 2 there is hardly any Ga near the surface of the CIGS layer where the junction of the solar cell is made. This surface portion is practically a CIS (CuInSe2) layer with a small bandgap (0.95-1 eV) and consequently solar cells fabricated on such layers display low open circuit voltages (typically in the range of 400-500 mV) and thus lower efficiencies. CIGS layers processed by evaporation techniques and yielding high efficiency solar cells typically have substantially uniform Ga distribution through their thickness and 20-30% Ga near their surface regions. Such devices give open circuit voltage values of over 650 mV, even over 700 mV. It is believed that when a metallic precursor film comprising Cu, In and Ga is deposited first on a base and then reacted with Se, the Ga-rich phases segregate to the film/base interface (or the film/contact interface) because reactions between Ga-bearing species and Se are slower than the reactions between In-bearing species and Se. Therefore, such a process yields compound absorber layers with surfaces that are rich in In and poor in Ga. When a solar cell is fabricated on such an absorber layer, the active junction of the device is formed within the surface region with a low Ga/(Ga+In) ratio as shown in FIG. 2. Therefore, whatever the mechanism of Ga segregation in two-stage processed CIGS layers may be, it is important to develop approaches that yield CIGS films with a surface Ga/(Ga+In) molar ratio in the 0.15-0.30 range.